Wireless power transfer has many industrial applications, and devices utilising wireless power transfer, such as wireless toothbrush chargers, wireless charging pads for mobile devices, and wirelessly charged medical devices implanted within the body, continue to grow in popularity.
Inductive power transfer (IPT) is an example of non-radiative wireless power transfer. In a typical inductive power transfer system, an alternating current passes through a transmitter coil. This causes the transmitter coil to produce a time-varying magnetic field. When a receiver coil is placed in the time-varying magnetic field, the magnetic field induces an alternating current in the receiver coil, which can then be used to drive a load. Thus, power is transmitted wirelessly from the transmitter coil to the receiver coil through the time-varying magnetic field.
When designing an inductive power transfer system, several factors need to be borne in mind, and several problems present themselves. In order to achieve efficient operation and maximum power throughput, it is generally required to operate the IPT system using a large magnetic field. However, design of the system can be restricted in this respect, for example by guidelines relating to exposure limits for electromagnetic fields set by the International Commission on Non-Ionising Radiation Protection (ICNIRP).
It is possible to use a power inverter to convert a DC signal to an AC signal in order to drive a transmitter coil in an IPT system. It is also possible to use a transistor as a switch within the inverter. When using a transistor switch however, two types of power loss can present themselves: conduction loss, and switching loss. The first is associated with the finite resistance of the transistor, whilst the second is associated with switching the transistor at non-zero voltage and non-zero current. This second type of power loss can be minimised using ‘soft-switching’ techniques, for example zero-voltage-switching (ZVS) techniques. ZVS involves switching the transistor on/off whilst zero voltage passes through the transistor.
With the above in mind, a problem with existing systems is that the magnetic field transmitted by a transmitter coil is dependent on the receiver load. For example, in a system with multiple devices each having a respective receiver load, the power available to any one device can be reduced if another receiver device moves closer to the transmitter coil. Also, introducing a new receiver device to the IPT system can reduce the power available to all the original receiver devices. The number, location and orientation of the receiver coils in the IPT system affects the effective resistive load of the transmitter coil, which brings about a change in the current passing through the transmitter coil. This in turn alters the magnetic field produced by the transmitter coil. This variation in magnetic field may cause the magnetic field to exceed ICNIRP limits and/or cause an unwanted reduction in maximum achievable range or power throughput. The change in current also causes increased power losses, and hence reduced efficiency of the IPT system, due to loss of ZVS operation.
It is desirable to provide an inverter for driving a transmitter coil which retains a high efficiency and which delivers a constant current to the transmitter coil, independent of the load. In providing such an inverter, it is also desirable to avoid or reduce the overhead of real-time circuit and system level control. It is also desirable to avoid switching losses which may occur whilst the transistor is being turned on and off.